1. Field of the Invention
The present invention relates to infrared detectors and specifically infrared detectors having improved properties with regard to impedance matching and detectivity.
2. Description of the Related Art
Most prior infrared detectors have either been photoconductive, PC, or photo-voltaic, PV, detectors. For most applications, these detectors must be cooled in order to obtain the best sensitivity. PV devices, which have a high input resistance and consequently a low power consumption, are suited to a monolithic input circuit, whereas PC devices typically require so-called "off-chip" circuitry to handle the lower impedance.
There are two temperature regions where different noise sources dominate the detector's performance. These two regions are referred to as the "detector limited" and "BLIP or shot noise" regions. The detector limited temperature region, typically at relatively higher temperatures, is where detector noise dominates the ultimate performance of the device, and the dominant noise source is usually noise associated with the fluctuations in the generation and recombination rates of the current carriers in the detector, referred to as g-r noise. The BLIP temperature region, above a critical temperature, is where the fluctuations in the scene flux (the intercepted optical radiation) contribute the major portion of the detector's noise. This is also referred to as "optical g-r noise" (in PV detectors, the optical recombination noise is insignificant in relation to the optical generation noise; for this reason PV detectors have an ultimately lower noise value by one over a square root of two than that of a PC detector). The value of the critical temperature, which divides the two regions, is practically the same for both PV and PC detectors.
Recent work in PC type devices has been directed at increasing the critical temperature, which divides the two regions of operation, and at the same time, increasing the impedance of the PC detector. One reference article is by D. L. Spears, et al. "HgCdTe resonant optical cavity IR detectors", Proceedings Of The IRIS Specialty Group On IR Detectors, volume 2, August, 1985, page 95. The first of the two obvious advantages with this type of detector is that you can increase maximum temperature of BLIP operation of the PC device. With an increased operating temperature, many advantages appear. Several advantages are a decrease in the cooling costs and cooling systems' weight. The second obvious advantage, resulting from a higher detector impedance, is the possibility that a PC can be interfaced to a monolithic input circuit, in a manner similar to that of a PV device.
If it is possible to reduce the volume of the PC device, while still maintaining a high efficiency in capturing the IR radiation, the performance of the device can be enhanced in the detector limited region, and the critical temperature can be increased. The detectivity of the device, or D* (which is a measure of the device's performance), improves as the reciprocal of the square root of the volume of the detection material, other parameters being constant. The most recent prior work involves a polarization-sensitive, filamentous PC device. This PC has been patterned to achieve a reduction in its material volume. The patterning is done in conjunction with the formation of a resonant optical cavity beneath the patterned PC to form a structure which is impedance matched to the free space IR radiation. An "impedance matched" structure such as this, maintains a high IR radiation collection efficiency, while significantly reducing the volume of the PC. This pattern was there described as a mesh-type or multiple-filament-type pattern.
Nevertheless, there is substantial incentive for further improvements. The impedance of such a photoconductive detector still does not match the output impedance of a photovoltaic type of detector; and, moreover, the structure is typically sensitive to polarization of the incoming radiation in the sense that one polarization of the incoming radiation can be more readily detected than any other. It would be desirable to overcome these continuing disadvantages and to achieve a device with still other improved properties, such as better manufacturability.